/+    tree
/:    /%%/    /%
  /^    [tip=marl sub=(map span marl) ~]
  /.    /;    (getall:tree %h1 ~)    /elem/
        /_    /;    (getall:tree %h1 ~)    /elem/
  ==
!:
::::
  ::
^-  (map path marl)
=-  [[/ (zing -)] ~ ~]
^-  (list marl)
[tip (turn (~(tap by sub)) tail)]